Known supporting devices for sonotrode heads are attached at a location with a minimum amplitude (what is known as zero point support). This can achieve a situation where as little ultrasonic energy as possible is removed via the supporting device, which ultrasonic energy is to be provided for the actual welding operation. An undesired temperature rise at the supporting device can also be avoided as a result.
For instance, U.S. Pat. No. 3,184,841 discloses, for example, a torsion sonotrode which can be excited to perform torsional oscillations with the aid of ultrasonic transducers. The working face of the annular welding tip runs perpendicularly with respect to the torsion axis. The sonotrode is supported by a mass at an oscillation node.
WO 02/061895 A1 is concerned with the connection of electric conductors, which can be achieved, for example, by way of torsion sonotrodes or longitudinal oscillators. The longitudinal oscillators can be mounted at an oscillation node.
EP 2 261 008 A1 discloses further devices and methods for ultrasonic treatment. A sonotrode is excited to perform torsional ultrasonic oscillations. It has a slot, into which an engagement member of an anvil is introduced, with the result that a weld gap is formed for two plastic films to be welded. In the region of an action section, the engagement member has a cradle edge which is adjoined by a wedge face. A supporting segment which is arranged either on the engagement member or on a groove bottom has a cone tip which lies in the direction of the torsion axis and forms a zero point contact which does not oscillate during welding.
The torsional sonotrodes which are disclosed in WO 2011/138404 A1 comprise projections which have working faces in the circumferential region. In each case one annular face which lies on a node line with regard to a wavelength of a natural oscillation of the torsional sonotrode is provided on both sides of the working faces. In one exemplary embodiment, a supporting device engages around said annular faces. Pressure forces are generated with the aid of a pressure device.
In the exemplary embodiment of WO 2012/069413 A1, a torsional oscillator is surrounded at a longitudinal position behind a sonotrode by a clamping ring which forms a bearing for the torsional oscillator. Said clamping ring is typically arranged at an oscillation node.
All said known supporting devices have the disadvantage, however, that the position of the oscillation node is dependent on the forces and torques which act on the sonotrode head. Generally speaking, an oscillation node can be displaced by several millimeters. This leads to undesired removal of the ultrasonic energy and the associated disadvantages which have already been described above.